FRANKFURT (Reuters) - German research firm GfK GFKG.DE said on Tuesday that Michael Dell, founder of the personal computer firm, had bought a 9.8 percent stake in the company.

GfK, which is in the process of being bought by KKR (KKR.N) in a deal valuing the company at 1.69 billion euros ($1.80 billion), said in a regulatory statement that the multi-billionaire had raised his stake from 6.45 percent last week.

The disclosure comes only days before a deadline for shareholders to tender their shares for the 43.50 euro per share offer from KKR expires on Feb. 10 at 2300 GMT.

As part of the deal, majority shareholder GfK Verein will hold on to its 56.46 percent stake.

KKR has said its offer is conditional to a least 18.54 percent of GfK shares accepting the offer.

Separately on Monday investment fund Primestone Capital has raised its stake in to 5 percent from 3.21 percent.